An optical imaging system provides an image of an object to a sensor. Thus an optical imaging system includes an image forming assembly (e.g., a lens, a mirror, etc), and a sensor (e.g., photographic film, a detector array or a CCD array, etc.). Optical imaging systems for objects which are not self-luminous also include an optical source for illuminating the object.
FIGS. 1a-c show various known configurations for providing illumination in an optical imaging system. The configurations of FIGS. 1a, 1b and 1c are back illumination, side illumination and beam splitter illumination respectively. In the back illumination scheme of FIG. 1a, a source 102 illuminates a back side of an object 104. Light from object 104 passes through an imaging assembly 106 and provides an image to sensor 108. In this approach, object 104 must transmit some fraction of the light provided by source 102. Microscopy of transparent objects is often performed in the back illumination arrangement of FIG. 1a. Other examples of back illumination include U.S. Pat. No. 4,988,188, U.S. Pat. No. 5,098,184, and US 2003/0095079.
FIG. 1b shows side illumination, where source 102 illuminates object 104 from the side. Radiation from object 104 passes through imaging assembly 106 and provides an image to sensor 108. Here “side illumination” indicates that source 102 is not in the way of light passing from object 104 to sensor 108. Unlike back illumination, side illumination is applicable to opaque objects. A very common example of side illumination in practice is reading a book by light from an artificial light source. Other example of side illumination include U.S. Pat. No. 6,222,677, U.S. Pat. No. 6,480,337, U.S. Pat. No. 6,712,471, and US 2002/0109774.
FIG. 1c shows beam splitter illumination. In this arrangement, some light from source 102 is deflected toward object 104 by a beam splitter 112. Light from object 104 passes through imaging assembly 106. A fraction of the light passing through imaging assembly 106 also passes through beam splitter 112 to provide an image to sensor 108. Beam splitter illumination is more complicated than back or side illumination, so it is usually reserved for cases where back or side illumination is inapplicable. One example of such an application is microscopy of opaque objects.
The arrangement of the illumination source has a strong effect on how compact an optical imaging system can be made. For example, the back illumination arrangement of FIG. 1a can be made quite compact, for example by positioning a flat-panel light source in close proximity to object 104. In contrast, side illumination and beam splitter illumination (i.e., FIGS. 1b and 1c respectively) tend to be less amenable to compact arrangements. For example, in side illumination, clearance must be provided to allow radiation from source 102 to reach object 104. This requirement for clearance tends to make it difficult to reduce the separation between object 104 and imaging assembly 106. Similarly, in beam splitter illumination, the beam splitter typically has a width on the order of the width of a region being imaged. Since the beam splitter is angled with respect to the optical path between object and sensor (i.e., the optical axis), it requires a amount of space along the optical axis comparable to its width. This factor makes it especially difficult to obtain a compact beam splitter imaging system that has a wide field of view.
The effect of the illumination arrangement on imaging system compactness can be appreciated by considering outlines 110 on FIGS. 1a-c. Outlines 110 closely surround all elements of the respective optical imaging systems, and schematically indicate what aspects of the arrangement are significant in terms of determining overall size. In all three cases, the illumination scheme significantly affects outline 110 (more so on FIGS. 1b and 1c than on FIG. 1a).
Since conventional provision of illumination for an optical imaging system is seen to undesirably increase system size, it would be an advance in the art to provide compact illumination for an optical imaging system. It would be a further advance in the art to provide compact illumination for an optical imaging system applicable to opaque objects. It would be another advance in the art to provide a wide-field optical imaging system having an object to sensor separation substantially less than the image width.